Yeah the account/posting thing could be better. I think if it’s public, then the poster should have the option of allowing comments by readers who do not have an account. The good news is, that Diaspora is free software and the goal is that users will becoming more and more distributed, running their own servers. And those who do will have the freedom to change those settings as they see fit.
As for Facebook, they have a…uh…”unique” way of interpreting the meaning of “public”.